There has been and is a considerable interest in the development of laser pulse generators which can produce radiation pulses of a very short duration, e.g. less than a nanosecond.
In the past, a number of different approaches have been used for generation of short pulses and following is a brief description of a few of these earlier methods.
One approach is described in the U.S. Pat. No. 3,979,694, issued Sept. 7, 1976 (Goldhar et al). This patent makes use of the laser-induced breakdown of gases. Laser pulses of the order of 0.1 nanoseconds are produced by means of a gas breakdown switch.
Other methods involved electro-optical pulse slicing and mode-locking techniques.
Excimer lasers are now well established as reliable, relatively efficient sources of high power radiation. An important feature of the excimer lasers is the broad amplifying bandwidth which they exhibit, e.g. the XeCl laser provides gain over a wavelength range of approximately 15 .ANG.. Amplification over such a broad spectral width suggests the possibility of generating extremely short pulses. As a result, a considerable effort has been devoted to the investigation of ultrashort pulse generation with excimer lasers.
Electro-optical pulse slicing for the production of subnanosecond excimer pulses has been reported in the publication by D. A. Jaroszynski and T. A. King, J. Phys. E: Sci. Instrum., 16, 862, 1983. However, this technique requires a fast, low jitter switch and, although pulses of 400 ps duration have been produced with the aid of krytrons, such devices are not suitable for high repetition rate systems.
Mode-locking techniques also have been employed for a generation of short pulses by excimer lasers. Both active and passive mode-locking techniques have been used. However, the simultaneous use of passive and active mode-locking in a XeCl oscillator has provided the shortest pulses (300 ps) reported to date, as seen in the publication by M. Watanabe, S. Watanabe and A. Endoh, Optics Letters, Vol. 8, p. 638, 1983. This technique, however, requires a laser medium which exhibits gain for more than 150 nanoseconds.
There has also been demonstrated the use of an excimer laser functioning as an amplifier or regenerative amplifier of pulses produced by an auxiliary mode-locked laser (see: Applied Physics Letters, 31, p. 747, 1977, by I. V. Tomov et al). Such systems, although they have produced picosecond pulses, require a sophisticated arrangement and are fairly costly.
Another novel approach for shortening the pulse from a laser is described in European Pat. No. 42521, published on Dec. 30, 1981, (Hon). This patent uses stimulated backscattering processes, such as Brillouin, Raman or Rayleigh, in a tapered waveguide to compress a laser pulse. An input pulse having a 20 nanosecond pulse width was compressed to a pulse of approximately 2 nanosecond.
Despite all these investigations, the need for a simpler generator for very short pulses is still great in view of possible applications in non-linear optics, photochemistry, plasma X-ray sources and dye laser pumping.
The present invention stems from observations that the stimulated Brillouin scattering process revealed very fast rise times for the backscattered pulses and that it also terminates very rapidly when the incident pulses produced optical breakdown in a scattering medium. The optical breakdown occurs when sufficiently powerful pulses are focussed at a gas/liquid interface.